1. Field of the Invention
The present invention relates to a model and method for calculating the temperature of a power MOSFET and, more specifically, to accurately calculating the temperature rise profile of a power MOSFET subjected to a defined current waveform or multiple waveforms.
2. Description of the Related Art
Important considerations in designing and employing a power MOSFET include the amount of power the power MOSFET can handle and the thermal effects generated by the power MOSFET during operation. A power MOSFET can be destroyed if it conducts too much current, because the power MOSFET generates excessive heat due to the extreme current. Increased temperature also can cause deleterious effects with respect to surrounding electronic components or other structures. Accordingly, it is important in any particular application to choose the proper power MOSFET, one that is capable of conducting the current flow for that application without generating excessive heat.
Although it is possible to manually calculate the temperature of a power MOSFET subjected to a simple current load, power MOSFETs typically are subjected to current loads having complex waveforms. Calculation of the temperature rise of a power MOSFET at a time tn subjected to a current having a complex wave form is difficult if not impracticable. As a result, power MOSFET selections for a particular application typically occur through trial and error.
Accordingly, a need exists for an automated method for calculating the temperature rise profile for a power MOSFET subjected to a defined current wave form.
The present invention provides a model and method for calculating the temperature rise profile for a power MOSFET subjected to a defined current waveform or a number of these same waveshapes. According to a preferred embodiment, the invention is embodied in the form of a computer program that operates on an industry standard personal computer.
Preferably, the program provides a graphical interface to a user whereby program data inputs can be provided by the user, and calculated data and graphical representations generated by the program model are presented to the user.
The program calculates and produces a graph or a tabular print-out of the temperature profile of the selected power MOSFET based upon data entered by a user. Program inputs relating to the power MOSFET include the device type and the device on resistance Rdson at the maximum device temperature (from the device data sheet), and various calculation data including the number of intervals required for the calculation and thermal resistance coordinates. The user also selects the desired current waveform from a library of waveforms. The program then calculates thermal resistance constants for the selected power MOSFET as a function of time, and generates an array of thermal resistance values for each waveform subdivision (the waveform subdivisions are determined by the number of intervals chosen). The instantaneous power values at each current/time subdivision then are calculated by the program and an array of powerxc3x97thermal resistance difference terms is generated for each time interval. These terms then are summed to generate the temperature rise profile, and the results are displayed graphically or textually.
Optionally, multiple current pulses of the existing waveshapes can be used.
Other features and advantages of the present invention will become apparent when the following description of the invention is read in conjunction with the accompanying drawings.